An aquifer is the porous material in which water is contained underground. It can consist of gravel, fractured rock, sand, silt or a blend of such materials and vary in thickness from a few feet to 1,000 feet or more.

Water is stored in the aquifer and withdrawn by well-pumping when needed.

GRUSP and NAUSP enable Arizona to maximize the use of its Colorado River entitlement and help the state reach its "safe yield" goal. Safe yield is the equilibrium between the amount of groundwater pumped from an aquifer and the amount recharged into it.